The present invention relates to a system for making-up steps lost by the stepping motor of a timepiece, comprising an oscillator used as a time base, a frequency-divider chain coupled to the oscillator, a shaping circuit for the pulses delivered by the divider chain, and a circuit for providing current driving pulses to the stepping motor under the control of the shaping circuit.
The principle of the invention is general, but it will be explained hereafter in the particular case where the stepping motor is of the Lavet type. As shown in FIG. 1, such a motor comprises a cylindrical permanent magnet 1', forming the rotor, inserted in a magnetic circuit 2' on which there is wound an excitation coil 3'. This motor requires bipolar control pulses as it is necessary to reverse the polarity of the magnetic circuit at each half turn of the rotor 1'.
Now, when such a motor misses a step, the following control pulse arrives with the rotor 1' already in its stable electromagnetic position, so that it will not turn. Thus, the motor will have lost two steps. This fault can become serious on watches where the time between two driving pulses is relatively large, for example, on watches which only comprise hour and minute hands.
To understand the principle on which the invention relies, let us first of all examine how the current used by the motor behaves in the following different cases.
FIG. 2 shows an oscillogram of the current consumed by a motor turning normally. At time t=0, a pulse of duration T.sub.D is applied to the excitation coil 3'. During the period a, the rotor 1' starts to turn and the current increases approximately exponentially. During the period b, the rotor 1' turns and creates a counterelectromotive force (e.m.f) which tends to reduce the current. During period c, the rotor 1' has reached its new position, the counterelectromotive force ceases, and the current increases up to the end of the driving pulse.
FIG. 3 illustrates an oscillogram of the current used by a motor in which rotor 1' is locked. In this case, the magnetomotive force (m.m.f) of the permanent magnet of the rotor 1' subtracts from the m.m.f. of the coil 3', and the iron, or core of the magnetic circuit 2' is not saturated. The rise of current is exponential, of the form EXP-(R.t/L), where the time constant L/R is relatively large with respect to the length of the driving pulse. FIG. 4 shows that the polarity of the magnet is opposed to that of the coil 3'. FIG. 5 shows that the ampere-turns of the magnet do not affect those of the coil 3', so that the inductance B is not very high.
FIG. 6 shows the oscillogram of the current of a motor, the rotor 1' of which is already in position at the time of the arrival of the driving pulse. It is to be noted that the current increases rapidly due to the fact that the equivalent inductance of the circuit is small. This is explained by the saturation of the magnetic circuit. FIG. 7 shows that the magnetic polarity of the rotor 1' is in the same direction as that of the core 2' and FIG. 8 shows that the ampere-turns of the magnet add to those of the coil 3', so that the inductance B is high, which produces saturation of the core.
Comparing the three cases discussed above and the oscillograms of current shown respectively in FIGS. 2, 3 and 6, it is to be noted that in the case of FIG. 6, the current measured, for example, two milliseconds after the start of the driving pulse has a value approximately two times larger than in the two other cases forming the subject of FIGS. 2 and 3. Consequently, a measurement effected approximately two milliseconds after the start of the driving pulse permits non-rotation of the motor to be detected. The duration of the measurement must be very short for the current consumption of the measurement circuit to be negligible.
The object of the present invention is to provide a system which detects the non-rotation of the stepping motor and delivers information to the logic driver permitting making-up of the lost steps.